The present exemplary embodiment relates to radiopharmaceuticals. It finds particular application in conjunction with a dose calibrator or radioisotope calibrator for positron isotopes, and will be described with particular reference thereto.
A dose calibrator (radioisotope calibrator) is a device used in nuclear medicine that measures the total energy of a specific radionuclide in units of Curies (Ci), millicuries (mCi), or microcuries (μCi). It includes a hollow, lead-shielded cylinder, into which radionuclides are lowered for measurement. Such devices can be programmed for specific radioisotopes, or adjusted for isotopes not preprogrammed. A dose calibrator is commonly used to obtain measurements of the total radioactivity of isotopes during or after manufacturing and prior to administration to patients undergoing nuclear medicine diagnostic imaging procedures or radioisotope therapy procedures. Regulatory authorities specify when a radioisotope dose calibrator will be used and the timing of required quality control checks (constancy, accuracy, linearity, and geometrical dependence).
Currently, a dose calibrator source standard is used for calibration of the dose calibrator. The calibration source includes a cylindrical vial comprising a predetermined amount of the radionuclide to be calibrated, together with a decay calendar, which allows a user to determine the amount of radionuclide present in the source at the time of testing. The current dose calibrator source standards are designed to closely approximate the geometry of manufacturing containers and unit dose radiopharmaceuticals dispensed by radiopharmacies and to meet the standards implied in CEI-IEC 61145 “Calibration and Usage of Ionization Chamber Systems for Assay of Radionuclides;” CEI-IEC 1303 “Medical Electrical Equipment—Radionuclide Calibrators—Particular Method of Describing Performance;” ANSI N42.13-1986 “Calibration and Usage of ‘Dose Calibrator’ Ionization Chambers for the Assay of Radionuclides;” and, 10 CFR 35.50 “Possession, Use, Calibration, and Check of Dose Calibrators.”
The current Dose Calibrator Source Standards are designed to approximate the geometry of a typical radiopharmaceutical multidose vial or radionuclide generator elution vial (10 milliliters volume to 30 milliliters volume, respectively) used for gamma emitting nuclides. However, Positron Emission Tomography (PET) devices employ positron-emitting radionuclides, such as Fluorine 18. The positrons collide with a subject under investigation, resulting in the emission of pairs of gamma rays, which are detected. PET imaging systems are widely used to diagnose cancer recurrences, metastases of cancer, whether an early stage of cancer is present or not, and, if cancer has spread, its response to treatment. PET is also used in diagnosing certain cardiovascular and neurological diseases by highlighting areas with increased, diminished, or no metabolic activity. This has lead to a demand for manufacturing and dispensing of PET radionuclides and radiopharmaceuticals in a vial or a syringe for unit dosing. For accurate measurements, dose calibrator source standards that approximate radioactivity volume, physical geometry, and radionuclide photon emission of the unit doses are desired. However, common positron emitting radionuclides are short lived and are not amenable to being incorporated into dose calibrator source standards. Accordingly, even though the PET device may incorporate highly accurate gamma detectors, it is often the case that changes in cancers over time are not observed because of lack of accurate calibration of the dose applied to the patient or tissue sample.
The current methods for calibrating PET emitters are based on theoretical calculations which have now been found to provide errors of about 20%.
There remains a need for a calibrator source standard and calibration method for simulation of PET radionuclides which can be used for calibration of a dose calibrator which can then be used to check the unit dose.